Ok, did another test and it doesn't seem that much faster but maybe a little. It's definitely different though. The way it comes out of development is different. The formic came in quicker with and then quite gradual. The acetic/vinegar hesitated then came in pretty quickly.
I kept all parameters the same as best I could.
This was in shade at f/8 11am, 10sec exposure. I had a clock to see how many seconds development was and tried to stop at the same point (I was looking at his eyes). It was 15s for the formic (top one) and 20s for the bottom one (vinegar). The results show that I probably should have done 15s for the vinegar as well.
Looking at an angle the density is much better for the formic. Look around the plates and you see difference in contrast and exposure. The vinegar developer (unfortunately not my normal dev) has that weird area that looks over exposed.
Anyway, I'm not sure what to do with this. It's completely usable but I don't know if there is a gain. In this instance I'd take the formic over the vinegar.
In those papers it sounded like the formic was better in low light. I'd say you can definitely see a difference in density. So perhaps you can get a better looking plate in low light with formic. Probably a little gain in exposure. I had done formic at 5 sec and felt it was a bit underexposed but stopped development probably at 20sec. Perhaps formic prevails with shorter exposure and longer development?
I went ahead and scanned the five second exposure with formic. I probably could have developed longer and got a similar result. a bit odd exposure on the right side.